GASTEROPODA. 



275 



The Air-breathing Gasteropods (Pulmonifera *) of 

 which the Slug and the Snail are familiar examples, 

 respire atmospheric air, ^yhich is alternately drawn 

 into and expelled from a cavity lined wdtli a most 

 delicate net-work of blood-vessels : this respiratory 

 organ ojDens externally on the right side of the body, 

 near the margin of the shell, below the collar of the 

 mantle. Some are terrestrial, others live in streams 

 or in sluggish stagnant waters, some are shelled, 

 others are naked. 



The Terrestrial Air-breatliing Gasteropods are at 

 once recognizable by their four tentacula or horns, 

 as they are commonly called. These appendages are 

 retractile, and the upper pair have eyes at their ex- 

 tremities. The mouth is armed with a broad cutting 

 tooth, and a w^onderfuUy-constructed tongue, studded 

 with innumerable microscopic teeth. Some are only 

 provided with an internal shell, while others have an 

 external shell spirally twisted. 



The Slug^s (Limax) are without any apparent shell ; 



Fig. 211. — snails amj slugs. 

 * Pulmo, a lung ; fero, to carry. 



